re: Slab question for building a house

Post tension slab will make the slab more rigid overall, causing less cracks. Assuming your house is slab on grade rather then on piles, it's probably unnecessary- though that's also assuming you have adequate bearing capacity for soil, which for SELA is often questionable.

It's really a geotechnical engineering question, rather than a structural one.

Could be that the builder doesn't know how to do it or is worried he won't get the job if you insist on it. Ask a geotechnical for a site visit and/or soil borings if you're concerned about it.